Largest hole in ozone layer ever

By Andrew Darby

This spring's Antarctic ozone hole has been declared the largest on record, an emphatic reminder that humanity's atmospheric harm is set to continue for decades.

The hole peaked at 29.5 million square kilometres, exceeding the previous largest in 2000 by 100,000 square kilometres, the World Meteorological Organisation said yesterday.

It was also the deepest recorded over eastern Antarctica, at one point leaving a layer of sky above Australia's Davis station with no ozone at all to guard against ultraviolet radiation.

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The roughly oval-shaped hole, four times the size of Australia, swung over South American towns and the Falkland Islands, and reached its greatest extent on September 25.

Its effects on people were limited by the sun's low angle at that time of year, and cloud. Scientists point to greater potential problems in the polar seas, where there is evidence excess UV radiation may be hitting the phytoplankton that underpins life there.

"Our recent studies of satellite images show that the amount of chlorophyll in the water, a measurement of phytoplankton, could be reduced by up to 65 per cent," said Dr Andrew Davidson, a marine microbiologist with the federal Environment Department's Antarctic division. He said growth in microscopic marine plant life could be inhibited by 6-12 per cent in waters covered by sea ice and lead to changes in some krill species' abundance.

A bulletin on this year's depletion said an area of the far South Atlantic, which is known as the breadbasket of Antarctica, was this month exposed to three to six times the normal amount of UV radiation.

This "is a region with large bioproduction compared to other regions of the globe and is hence particularly vulnerable to elevated UV radiation", the bulletin said.

Around this time, the hole's depth reached a low of 85 Dobson units over the East Antarctic Ice Sheet. Before the annual hole developed in July, there were about 300 of the Dobson units - an ozone prevalence measurement.

Extremely cold temperatures over winter, down to minus 93 degrees in the upper atmosphere, were behind the expansion of this year's hole. An Antarctic division atmospheric scientist, Dr Andrew Klekociuk, said the cold could be linked to climate change. Some global warming scenarios point to a balancing cooling effect in the upper atmosphere.

Bans on chlorofluorocarbons have stemmed the ozone damage. But NASA scientists recently said the ozone layer may not recover until about 2068.